This invention relates to load pull testing of microwave power transistors using automatic microwave tuners for synthesizing reflection factors (or impedances) and match the transistors (device under test, or DUT) at the input and output at the fundamental and harmonic frequencies.
Modern design of high power microwave amplifiers, oscillators and other active components used in various communication systems requires accurate knowledge of the active device's (microwave transistor's) characteristics. In such circuits, it is insufficient and inaccurate for the transistors operating at high power in their highly non-linear regions and close to saturation, to be described using analytical or numerical models only. Instead the devices must be characterized using specialized test setups under the actual operating conditions.
A popular method for testing and characterizing transistors for high power operation is “load pull” and “source pull”. Load pull or source pull are measurement techniques employing microwave impedance tuners 2, 4 and other microwave test equipment, like signal sources 1, an RF (Radio Frequency) load 5, control computer 6 and digital connections 7, 8, 9 between the computer and the tuners and test equipment. The microwave tuners 2, 4 in particular are used in order to manipulate the RF impedance conditions under which the DUT 3 is tested (FIG. 1).
Electro-mechanical tuners (see ref. 1) are used in most cases for high power load pull testing, because they have several advantages, such as long-term stability, higher handling of RF power, easier operation and lower cost, compared to other type of tuners such as electronic and active tuners. FIG. 2 shows a front view and cross section of an automatic electro-mechanical tuner using the “slide-screw” tuning concept; a slotted airline (slabline) 215, with two RF connectors 25, 26 at both ends is embedded in a solid housing 216, which also comprises a mobile carriage 28 and means for horizontal drive, typically a lead screw 217; the carriage 28 slides smoothly on polished and grounded shafts 219. The carriage 28 comprises electrically driven 20a stepper motor 20, which is powered by a control computer 6, running appropriate software and controls the movement of a precise vertical axis 21. At the bottom end of the axis 22 an appropriate clamp 22a holds the RF probe 23 and secures its exact and repeatable positioning very close to the center conductor 24 of the slabline 215. Moving the probe 23 closer to the center conductor 24 increases the amplitude of the reflection factor, and moving it along the axis of the slabline 215, at constant distance from the center conductor, controls its phase. The vertical probe control is shown in FIG. 3.
A cross section of the ‘slide screw’ showing slabline, probe and vertical tuning mechanism is included in FIG. 3; in this configuration adjustable metallic obstacles (probes or “slugs”) 30 are inserted into the transmission media of the tuners, which is a slotted coaxial or parallel plate airline (slabline) 31; the capacitive coupling between the vertical probe 30 and the central conductor 32 of the slotted airline (slabline) creates a wideband RF reflection factor (Γ) for the electromagnetic waves travelling along the slabline, of which the amplitude can be adjusted by inserting (or withdrawing) the probe 30 further into (or from) the slabline and reducing (or increasing) the gap between the probe 30 and the central conductor 32 and therefore increasing (or reducing) the value of the capacitance between the center conductor 32 and the probe 30.
The probe 30 is held and guided by the vertical axis 33 of the tuner and is moved vertically 34 by the axis 33, which is driven by a vertical lead screw and computer controlled stepper motors, known in prior art (FIG. 3 in ref. 2), FIGS. 2, 3.